Possible Clues:

• Line between centre and circumference of a circle
• Sounds as if the top eight makes a gap over us
• Half way across you find a bone here
• It runs from the centre to the circumference
• Half a circle's diameter
• Bone from old Persian king whose top half is dug up
• Bone of forearm - distance from centre
• The distance from a centre
• It's half the length of the diameter
• Line from the centre to perimeter of a circle